1. Field of the Invention
The present invention relates to a semiconductor structure and a method for making the structure, and more particularly to a semiconductor structure which comprises a self-aligned bipolar transistor where the emitter self-aligns to the base contact diffusion using an extrinsic base region, and a polysilicon capacitor, and a method which makes them simultaneously.
2. Discussion on of the Prior Art
It is well known that a key element in fabricating high precision CMOS analog integrated circuits is the use of a polysilicon to polysilicon capacitor, with a silicon dioxide--silicon nitride-silicon dioxide (O--N--O) dielectric region between the polysilicon electrodes. Such a structure provides high linearity and good matching characteristics. However, the fabrication of the O--N--O region requires at least three thermal heat cycles.
In order to fabricate a high performance bipolar transistor, it is well known that self-alignment techniques must be utilized. See, e.g., S. Sze, VLSI Technology, pp. 504-06 (2d ed. McGraw-Hill 1988); W Ko et al., A Simplified Fully Implanted Bipolar VLSI Technology, IEEE Transactions on Electronic Devices, Vol. ED-30, No. 3, pp. 236-39 (March 1983). A self-aligned extrinsic base, for example, is formed by opening a window in a silicon nitride layer after the formation of the field oxide structure. This allows independent optimization of the base doping level for optimum transistor action, consistent low base resistance, and good ohmic contact to the base.
In one prior art bipolar transistor, a mesa structure comprising of a silicon dioxide region with a silicon nitride region thereon, is formed on an extrinsic base, with a base contact region to one side and an emitter region to the other side.
In the fabrication of a typical BiCMOS device, the bipolar portion is formed first, then the CMOS portion. In the event the CMOS portion requires a precision capacitor element, the heat cycles required to form the O--N--O region would cause the extrinsic base of the bipolar device to undergo additional diffusion. For this reason, it has been very difficult to use a self-alignment scheme to form a bipolar transistor which includes the polysilicon O--N--O polysilicon capacitor process module, without compromising the performance of the bipolar transistor.
In addition, the extra heat cycles required to form the O--N--O capacitor would result in significantly different bipolar transistor characteristics, between processes with and without the O--N--O capacitor process module.
Therefore, it would be desirable to maintain a minimum diffusion of the extrinsic base when forming a bipolar transistor which includes the polysilicon O--N--O polysilicon capacitor process module.